In order to ensure safety and secure for a structure such as a high-pressure pipe line, a water and sewage plumbing, a high speed railway, a long span bridge, a high rise building, a large passenger aircraft or a car, non-destructive inspection techniques have been researched and developed. As the non-destructive inspection of the structure, the crack detecting method by penetration inspection and the crack detecting method by ultrasonic inspection are exemplified (for example, refer to a non-patent literature 1). FIG. 9A shows an outline of the crack detecting method by penetration inspection. The crack detecting method by penetration inspection is a method to apply a fluorescent material 2 to a member 1 which is a component of a facility, and to make the fluorescent material 2, which penetrates into a crack 3 corresponding to a defect of the structure, luminous, and to check the crack 3 by inspector's eyes. Since a check based on the method can be carried out with ease, the method is used frequently. FIG. 9B shows an outline of the crack detecting method by ultrasonic inspection. The crack detecting method by ultrasonic inspection is a method to use an ultrasonic transducer 4 which is an electromechanical converter, and to identify a crack 3 of a member 1 by radiating ultrasonic waves to the member 1. The method uses a property that acoustic impedance at a location, at which the crack 3 is caused, is different from acoustic impedance at a normal location. Identification of the crack 3 of the member 1 is carried out through receiving a reflected wave, which is generated by reflection of an ultrasonic wave signal propagating through the member 1 and which is generated at the location of the crack 3, by use of the electromechanical converter.